Manufacturing profiled parts obtained by extruding organic material and reinforcing fibres, such as glass fibres, through a die plate and manufacturing objects obtained by injecting these same constituents into a mould encounter the same difficulties when the said constituents are mixed: firstly, the mixture of the organic material and the reinforcing fibres must be as homogeneous as possible before the moulding operation; secondly, it is desirable that the reinforcing fibres impart the best mechanical properties to the composite product manufactured and, to this end, it is in particular desirable to avoid the fibres breaking to too great an extent.
It is known to perform this mixing operation using an extruder which is formed of a heated cylinder in which a lifting screw is rotated by means of a motor. At the top of one of its ends, this cylinder comprises a hopper of which the base opens directly above the lifting screw. The extruder is fed with organic material and glass fibres via this hopper.
The organic material and the reinforcing fibres, for example glass fibres, can be introduced simultaneously into the hopper in a number of ways:
The organic material is stored in the form of granules in a hopper disposed above a conveyor belt of which the velocity of advance is adjustable; the glass fibres are stored in the form of cut fibres in a further hopper, disposed above a further conveyor belt of which the velocity of advance is also adjustable. These belts discharge their contents into the extruder feed hopper. This method requires the use of heavy or bulky metering devices to maintain the respective proportions of organic material and cut fibres constant.
A second method consists in mixing beforehand the organic material and the cut fibres. This mixture, discharged into a hopper, is conveyed to the extruder feed hopper by a conveyor belt. This method also requires additional equipment for mixing the two constituents.
The extruder can also be fed directly with granules comprising glass fibres coated with organic material. These granules can be obtained by different processes. Thus, patent application EP-A-O 393 532 recommends impregnating the glass fibres under pressure and then cutting them into pieces.
A variant is mentioned in patent application EP-A-O 367 661. It consists in coating the filaments of a composite fibre with an organic finish before it is exposed to actinic radiation. The aim thereof is to impart cohesion to the composite fibre which can then be cut into sections by a cutting machine.
Introducing the organic material and glass fibres into the extruder at two different points is also known. The organic material is introduced in the form of granules, upstream of a double lifting screw, in the same manner as in the first process described above. The glass fibres are introduced into the extruder downstream of the area in which the organic material is mixed and melted. They are introduced in the form of continuous fibres, extracted from one or more rolls disposed on a creel. A process of this type is described in U.S. Pat. No. 3,304,282 for example.
The factor common to these various processes is that they are performed by very long extruders equipped with profiled screws which in some cases are complex.
The aim of the single or double lifting screw upstream of the extruder is to mix and melt the granules of organic material which may or may not comprise reinforcing fibres. In order to fulfill this function, the screw can have different profiles depending on the grain size of the organic material used. The helix angle of the screw thread, the depth of the thread, the shape of the screw core, cylindrical or conical, the screw pitch, etc. are all factors which determined the degree of compression and shearing of the organic material.
In addition to the length and complexity of the screw used, it should also be stressed that a large amount of energy is needed to obtain a homogeneous molten mixture.
When the reinforcing fibres are introduced into the extruder mixed with the granules of organic material, these fibres are also subjected to shear stresses causing the material to melt, which results in their breaking up.
This disadvantage could in part be avoided if the reinforcing fibres were introduced into the extruder in the form a continuous fibre, downstream of the area in which the thermoplastic organic material is melted. The reinforcing fibres must in turn be subjected to intense mixing in the molten material in order to be distributed homogeneously in the said material. This operation also causes large-scale breakage of these fibres.